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A PROMISED TRANSIT OF VULCAN.

ACCORDING to the well-known astronomer and mathe- matician, M. Oppolzer, the Sun will probably rise on the morning of March 18th with the planet Vulcan in transit across his face. M. Oppolzer has been endeavouring to reconcile the various observations of round black spots which have been made from time to time (without exception by inexperienced observers, as it happens), though Leverrier and other astronomers of skill have deigned to take such observations into account. Leverrier compared the well-known observation of a black spot in transit, made by Lescarbault on March 26th, 1859, with several other accounts of round spots, and by combining them together, de- duced several orbits which could be reconciled with these observa- tions. If he had only deduced one, the result would have been on the whole more satisfactory; but he obtained no less than four, nor did he succeed in showing that there might not be others. In fact M. Oppolzer now comes forward with an entirely new orbit, the elements of which differ greatly from any of those found by Leverrier, and fairly satisfy no less than eight of the best at- tested records of round black spots upon the Sun's face. This is not all. He points out that with the small inclination which Lesearbault's Vulcan has—always assuming it to exist—and a periodic time of from fifteen to eighteen days, the supposed planet would so often cross the Sun's face, that it becomes easy to account satisfactorily for eight observations of round spots, in a number of ways, all of which save one must be wrong, while probably not one may be right. Besides, while with such a solution it becomes very easy to account for any number of observed round spots, it becomes exceedingly difficult to under- stand why a planet following such an orbit has not been seen a great deal oftener, and especially to understand the perplexing circumstance that no experienced astronomer has ever seen one of these mysterious spots. True, Weber, who saw such a spot at Pechele, in China, was a tolerably skilful observer ; only unfortunately it turned out that the spot he saw was an ordinary sun-spot, which was seen and unmistakably recognised as such at Madrid, and photographed at Greenwich. Schwabe, who observed the Sun on every clear day for fifty successive years, never saw a round black spot,—or perhaps it would be more correct to say that he never mistook such a spot for a planet. Carrington, Secchi, De in Rue, and in fine, all the most systematic and patient students of the Sun, have failed to see these round bodies. It has been given only to the inexperi- enced observers, using small and often ill-mounted telescopes, and mostly unable to interpret what they saw, who have been enabled to announce to the scientific world their recognition of bodies "which cannot have been ordinary sun-spots," because either they seemed so black and round, or else they changed in position more rapidly than a spot would. Lescarbault's obser- vation was of this kind. He saw a round black spot close to the upper edge of the Sun's disc, observed that it moved rapidly towards the west, and when (months afterwards) Leverrier in- quired about this object, Lescarbault stated that he had watched it pass off the Sun's face, and showed the record of the time when this happened. An inexperienced observer might have seen a spot where Lescarbault saw his, might have mistaken for a westerly motion of the spot the ap- parent westerly displacement due to the way in which, as he approaches the west, the Sun throws westwards the part of his disc which had been uppermost when he was due south, and might have calculated when this supposed motion would carry the spot off the Sun's face, subsequently mistaking the record of the result of his calculation for the record of something he had actually witnessed. Now Lescarbault most certainly was an inexperienced observer, even when Leverrier called on him, and we have no evidence that the little he then knew about observation had not been acquired after the date of his supposed discovery. So little did Lescarbault know of the mathematics of astronomy, that he was unable to solve the comparatively easy problem (it is solved, as an easy example, in Proctor's" Geometry of Cycloids ") of determining from the time of the supposed planet's transit the period in which it circles around the Sun. It would have been very natural for him to have made such a mistake as we suppose, though it perhaps would not be easy for him now to admit the possibility of his having done so. We should, then, also understand what even Leverrier, in the fullness of his sub- sequent faith in Lescarbault, found mysterious and annoying,— namely, the strange silence of Lescarbault for nine months after a discovery so remarkable. Lescarbault explained that he was waiting to see the planet again ; but he must have been strangely ignorant of the history of solar observation, to expect such good-fortune. Now, if at the time of the observation he had only calculated when a certain apparent motion would have carried off the supposed planet, and had not witnessed the phenomenon—perhaps even had waited in vain expectation of witnessing it until after the calculated time, and had then per- ceived that the motion was apparent only, not real—he might readily have let the whole matter pass from his memory. When the matter was recalled to him by Leverrier's announcement that there probably are planets between Mercury and the Sun, the recollection of it might have been only imperfect; and we know well how readily an imperfect recollection might gradually have grown into the story related to Leverrier, without the slightest intentional bad-faith on Lescarbault's part.

It is certain, at any rate, that no astronomer who weighs the full evidence in this matter can believe in Lescarbault's Vulcan. It may readily be shown that at a total solar eclipse occurring in June or July, or again, at such an eclipse in December or January, the supposed planet could not be invisible, no matter in what part of its orbit it might be ; even though it were in inferior conjunction, it would still be visible, because of the inclination of its orbit to the plane in which our earth travels. At total eclipses occurring at other times, Vulcan would be a conspicuous object, unless chancing to be in one particular part of its path. Among the numerous total eclipses of the Sun which have been observed, there must have been many at which such a planet would not only be conspicu- ous, but would be far the most conspicuous star in the heavens. Yet it has never been seen, even when astronomers have made special search for an intra-Mercurial planet. The objects seen during the eclipse of last July by Professors Watson and Swift have been supposed to be intra-Mercurial planets, of which one may have been Lescarbault's Vulcan. But they were all full- disked bodies, and the largest shone no more brightly than a third-magnitude star. Now it is quite certain that such a

planet as Lescarbault supposed he saw, when seen full- disked, would be far brighter than the brightest first-magni- tude star. It is equally certain, as has been shown in the current number of the Contemporary Review, that Lescarbault's Vulcan could not appear as a full-disked body where these sup- posed small planets were seen, or anywhere near that region of the sky, It may indeed be worth while to mention that if, as many astronomers believe, the objects seen by Watson and Swift were really planets, they could only be seen in transit with very powerful telescopes ; and not only so, but that a special search for them on the Sun's disc, with very powerful telescopes, could not fail to be soon rewarded with success.

If, despite such considerations as these, our readers still believe in Lescarbault's Vulcan, and consider that Oppolzer's new orbit may possibly be correct, it may interest them to learn when the promised transit is expected to begin or end. Oppolzer is more merciful than Leverrier, who used to tell the astronomers of both hemispheres to keep a watch during such and such days. If Oppolzer's orbit is correCt, all that astrono- mers will have to do will be to direct their telescopes on the Sun as soon after sunrise next Tuesday as he attains a convenient height. For the transit is to begin at 5h. 14m. a.m., or before sunrise, and to end at 10h. 42m. a.m., when the Sun will be at a convenient altitude for observation. We shall be interested, and to say the truth, we shall be yet more surprised, to hear that such observations as M. Oppolzer recommends have been rewarded with success. One circumstance is fortunate. The failure of observers to see Vulcan when Leverrier invited them to search for it, was only to be interpreted as affording nega- tive evidence; the planet might exist, even though many such observations failed. It is otherwise with the observations M. Oppolzer suggests. If no planet is seen, it will be certain that M. Oppolzer's Vulcan has no objective existence.